We present here the synthesis of spherical polyelectrolyte brushes by photo-emulsion polymerization with a solid polystyrene core (diameter 100 nm) onto which chains of vinylimidazolium-type poly(ionic liquid)s have been densely grafted. The as-synthesized brush particles were employed as nanoreactors for the generation and immobilization of metal nanoparticles. Transmission electron microscopy characterization shows that Au and Pd nanoparticles with diameter of 2.1 +/- 0.2 nm and 2.5 +/- 0.3 nm are homogeneously embedded inside the PIL brushes, respectively. The study of swelling behavior of the brush particles before and after metal deposition indicates obviously ion-specific effect. The composite particles exhibit a long-term colloidal stability in aqueous solutions as well as in ionic liquids. Catalytic activity of the as-synthesized metal nanocomposite particles is investigated by using the reduction of 4-nitrophenol with NaBH4 as a model reaction, which can be compared directly with reported systems. In addition, it is found that the rate constant k(app) of PIL-metal nanocatalyst could be modulated by salt concentration. (C) 2011 Elsevier Ltd. All rights reserved.